1. Field of the Invention
The present invention relates to a method for forming wiring on an insulating resin layer to constitute an inter-layer insulating layer of a multi-layer printed wiring board.
2. Description of the Related Art
Manufacture of a multi-layer printed wiring board constituting a printed wiring board, a semiconductor package (build-up board, package), a thin-component built-in board, a package-on-package (POP), etc., requires formation of wiring on an insulating resin layer serving as an inter-layer insulating layer.
As a representative example, in a case where a wiring layer made up of a metal such as Cu is formed on an insulating resin layer by electroplating in the manufacture of a build-up package, etc., a desmear process is generally performed for roughening the surface layer of a build-up resin by immersing it in a permanganic acid aqueous solution. The desmear process is originally made to remove machining scraps (smear) inside a via hole formed by laser machining, etc. Since the desmear process has a strong fluxing action on a resin, this process is applied for roughening the surface of a resin. When a wiring layer including an electroless-plated Cu layer (seed layer) and an electroplated Cu layer formed by a semi-additive process is formed on the roughened surface of the resin, unevenness of the roughened surface of the resin exhibits an anchor effect thus ensuring joining strength between the resin and the wiring.
However, in recent years, in semiconductor packages, etc., that use a multi-layer printed wiring board, in order to improve some characteristics, silica content of an insulating resin is increased so as to decrease thermal expansion coefficient. The increase of the silica content causes a drop in the desmear property of the resin, which makes it difficult to roughen the resin surface, resulting in insufficient joining strength between a resin and wiring.
Similarly, a prepreg material formed by impregnating a resin with a glass cloth is hard. Thus, it is difficult to roughen the prepreg material by the desmear process, and it is not possible to ensure joining strength with the wiring formed using the semi-additive process. Thus, a wiring formation by a subtractive method using a copper-foil-attached prepreg is mainly used.
Wiring formation by the subtractive method is not suited for finer design, and has limitations to finer design of the next generation.
On the other hand, a method has been practiced in the related art for roughening the resin surface not by using the desmear process, and ensuring joining strength between a resin and wiring in wiring formation by the semi-additive process (for example, refer to JP-A-2003-304068, in “Back Ground of the Invention”, especially paragraph [0007]). This related art method will be described referring to FIG. 1. For convenience, the method is described just for one surface of a board, however, processing may be performed on both surfaces of a board in parallel.
As shown in FIG. 1-(1), a glass-epoxy resin substrate 10 is provided, a base-layer wiring layer 14 made up of a copper foil being formed via an insulating resin layer 12 on the surface of the glass-epoxy resin substrate 10.
Next, as shown in FIG. 1-(2), a thermosetting resin film 16 in a semi-cured state and a copper foil 18 are laid one over the other and pressed with heat (hot press) on the substrate 10. On that occasion, a mat surface (roughened surface) of the copper foil 18 is overlaid on the surface of the semi-cured resin film 16. To the top surface of the resin film 16 in semi-cured state against which the mat surface of the copper foil 18 is pressed is transferred the unevenness of the mat surface to form a roughened surface R. Thus, an integral structure is obtained where a substrate 10 including an insulating resin layer 12 and base-layer wiring 14, an insulating resin layer 16 made up of a thermally cured resin film, and a copper foil 18 are laminated.
Next, as shown in FIG. 1-(3), the copper foil 18 is removed by etching. This exposes the roughened surface R of the insulating resin layer 16. The roughened surface R generally has the roughness Ra of the mat surface of the copper foil being equal to about 2.0 to 4.0 micrometers. The mat surface exhibits a mechanical anchor effect between the mat surface and the wiring layer formed by plating on the copper foil so as to ensure a high joining strength.
As shown in FIG. 1-(4), a via hole 20 is made in a predetermined part of the insulating resin 16 by laser beam machining, etc.
Next, as shown in FIG. 1-(5), smear in the via hole 20 is removed by a desmear process. The unevenness of the roughened surface R is smoothed by the desmear process to form a less roughened surface R′ rather than that the top surface of the insulating resin layer 16 is roughened by the desmear process.
Next, as shown in FIG. 1-(6), an electroless-plated copper layer 22 as a continuous layer is formed on the entirety of the exposed surface above the substrate 10, the exposed surface including the top surface [less roughened surface] R′ of the insulating resin layer 16, side surface of the via hole 20 and the top surface of the base-layer wiring layer 14 being exposed at the bottom of the via hole 20.
Next, as shown in FIG. 1-(7), the electroless-plated copper layer 22 is used as a power feeding layer to form wiring 24 made up of an electroplated copper layer by a semi-additive process.
The wiring 24 thus obtained has a smaller mechanical anchor effect compared with the original roughened surface R because an interface between the wiring 24 and the insulating resin layer 16 as a ground layer is a less roughened surface R′. Thus, a large anchor effect that should have been obtained by the transfer of the unevenness of the copper foil mat surface can not be obtained, resulting in an insufficient joining strength between the resin and the wiring.